Why would a emulated one be faster than one ported from the official source?

The mac version is a commercial game and considerably more optimal than the basic ports the Amiga got from the generic open source codebase. In fact there are lots of games where the mac version runs better than the equivalent Amiga versions and even pc versions with support for higher resolutions. Even with the overhead of emulation, the mac versions can run better than the Amiga native versions.